It is known that certain power tools such as portable chain saws or circular saws may sometimes undergo violent or sudden sharp movements or "kick-back" during use which may cause serious injury to the operator or damage to the device if the cutting members of the power tool are not stopped rapidly.
There exist many safety devices having a brake employing a brake band actuated by a return-spring-loaded safety bar or lever wherein the safety lever is maintained in the spring-loaded condition by a pawl. The pawl is intended to release the lever as soon as said lever executes a rotary movement caused by impact of the hand or wrist or arm of the operator holding the saw. The result of this impact is that the spring transmits a rotary movement to the lever which thus actuates the brake and arrests the movement of the saw chain. Devices of this type are, for example, described in U.S. Pat. Nos. 4,059,895; 3,964,333; 4,121,339 and 3,776,331 and in German Offenlegungsschrift Nos. 2922573/4 and 2632304. These devices require that the safety lever be positively rotated in order to disengage the locking pawl and thus actuated the brake. In the event that the operator improperly holds the chain saw, it is possible with a kick-back movement that the safety lever will not impact the hand or arm of the operator to be positively rotated thereby and the chain saw will continue to operate with possible resultant injuries to the operator.
U.S. Pat. No. 4,121,339 and German Offenlegungsschrift No. 2922573 and U.S. Pat. No. 3,923,170 indicate that the guard member forming the safety lever may itself also form an inertia mass during kick-back to actuate the brake by virtue of the inertia of said guard member being equivalent or proximating to the manual force required to activate the brake when such guard lever strikes against the arm or wrist of the operator. German Offenlegungsschrist No. 2621818 discloses a guide bar for the chain which is pivotable in the casing so as to displace a lever which in turn trips a safety lever which is also arranged to contact the hand or wrist of an operator during kick-back o actuate the brake. In this device the same or a similar force is required to trip the safety device and effect locking as the manual action and this is substantial. This force together with the speed of reaction and power involved therein is not considered adequate for desired safety. German Offenlegungsschrift No. 2632304 also discloses an arrangement wherein a manually actuable safety lever is operable to disengage a clutch. It is desirable to have a safety device with sufficient sensitivity of operation which has a guard member having the dual function of being operable by striking against the hand or wrist of the operator and also whose own mass may provide sufficient inertia during a kick-back to activate the brake whilst also permitting normal daily use wherein the guard member may come into contact with objects during handling without unnecessarily activating the brake. To overcome this disadvantage, braking devices have been proposed which act immediately as a consequence of a sharp movement of the portable saw when the chain saw moves sharply in the direction of the operator's body and also when the safety lever does not receive an impact from the hand of the operator.
For example, a somewhat complicated and less efficient arrangement is disclosed in U.S. Pat. No. 3,785,465 wherein a safety lever is provided to impact the hand of the operator to cause, by electro-mechanical means, a locking arm to disengage the drive to a centrifugal clutch and to apply a brake to the clutch drum. An electrically operated, acceleration sensitive transmitter is also referred to as being arranged in the chain guide bar to energise and electro-magnet when the guide bar is subjected to kick-back and to similarly stop the chain by applying the brake. No details of the nature of the transmitter are given and it is considered such device, apart from being complicated to manufacture, would not produce the desired sensitivity, reliability and speed of reaction of the safety means, nor could such device be produced in economic manner.
U.S. Pat. No. 3,937,306 discloses a chain saw having a safety lever with spring means actuable to tighten a band brake to stop the chain upon contact with the operator's arm or wrist and further discloses a resiliently mounted carrying handle or front stirrup adjacent said safety lever and having a projection which with sufficient kick-back action also acts to operate the safety lever. This arrangement is not considered readily practical since it is difficult to provide the desired resilience of mounting of the front stirrup which permits such to be used as a supporting handle in daily use and which also may be resiliently deflected to activate the brake in desired manner. Further, it would appear resilient displacement of the supporting handle or stirrup to the brake activating position may well be dependent upon the operator's manual pressure on said supporting handle in addition to any inertial forces which presupposes the operator has maintained his grip on the device with kick-backk which is not necessarily the situation and which does not ensure adequate safety.
U.S. Pat. Nos. 3,485,326 and 3,485,327 disclose a chain arresting element slidable on an inclined surface upon a kick-back occuring so as to engage with the chain or with a drive drum and whereon said displaceable members are retained until subject to a kick-back force by magnetic means or by a spring bias. U.S. Pat. No. 3,485,327 also discloses a spring biased pawl engageable in cogs on a drum secured to the drive shaft to thereby arrest the chain movement. It is not considered in certain circumstances that these devices will be adequate as regards the speed of operation and effectiveness of stopping and, furthermore, these devices do not also include a safety lever operable by striking against the hand or wrist of the operator. U.S. Pat. No. 3,397,795 J. L. Dooley discloses an arrangement having a manually operated safety lever which is operable to disengage a clutch and thus remove driving torque from the cutting chain. In addition to this safety feature, the guide rail or bar around which the cutting chain is rotated is mounted so as to be displaceable by way of its inertia upon being subject to kick-back action so as to disengage clutch means and thereby remove the driving torque from said cutting chain.
U.S. Pat. No. 3,974,566 discusses various considerations involved in achieving desired safety control and makes mention of the various prior arrangements. U.S. Pat. No. 3,974,566 does not disclose a safety lever acting to apply a brake to the chain upon contact with the hand or wrist of an operator during kick-back, but rather achieves a braking effect by way of an inertia mechanism independent of operator-manipulation and operable in response to adverse conditions, such as chain saw kick-back, to trigger the release of stored energy developed by operation of said chain saw. This is done for the purpose of releasing a spring, thus actuating a safety brake mechanism; the brake applying means comprising said spring is retained in the inoperative position by pressurized gas within a chamber, which gas is supplied thereto by conduit means from the interior of the crank case of the internal combustion engine.
Referring now to U.S. Pat. No. 3,974,566, this arrangement of inertia responsive means (59), including a rotor means (60) and having an eccentric weight (62) operable to cause the rotor (60) to undergo rotary movement relative to a vent (55) of the pressure chamber (49) in response to a kick-back, and thus the inertia of the rotor (60) including eccentric weight (62) will open vent means (53) and depressurize the chamber (49) to permit the spring (34) to activate the brake.
These prior devices utilizing an inertia system are too complicated and therefore do not result in economic manufacture and, furthermore, do not operate within the required time limits for safety, or the sensitivity thereof is not sufficient. In many of the devices, the force required to be exerted by the component acting as the inertia mass to activate the brake means is too great to permit rapid actuation thereof upon occurrence of a kick-back with desired sensitivity and within the desired time limits.